1. Field of the Invention
This invention pertains to a temperature control system for controlling the flow of a cryogenic fluid to a housing disposed around a fluid transmission line to control the formation and maintenance of an ice plug within the interior of the transmission line in connection with installation, testing, modification or repair of the line.
2. Background Art
In the installation, testing and modification of relatively large diameter fluid transmission lines, a unique process has been developed for forming temporary plugs in the line to permit pressure testing, as well as to prevent loss of fluid from or ingress of fluid into the line during installation and repair thereof. U.S. Pat. Nos. 3,827,282; 4,112,706 and 4,220,012 to B. D. Brister pertain to various inventions related to the formation of a plug by freezing a quantity of liquid at a predetermined location within a fluid transmission line. The inventive concept disclosed in the aforementioned patents basically comprise the provision of a housing disposed around the exterior of the transmission line and forming a closed chamber through which a cryogenic fluid is circulated to freeze the slug of liquid within the interior of the transmission line itself in the immediate vicinity of the housing. The liquid to be frozen may be the liquid being transmitted by the line or a slug of easily frozen liquid, such as water, may be introduced into the line between two spaced apart pipeline pigs and pumped to the point at which the plug is to be formed. Other methods include the provision of a pipeline pig having a sealed chamber filled with liquid which expands on freezing to form the temporary seal at the predetermined point in the line.
It is important, in the formation of temporary plugs in fluid transmission lines in the manner described, that careful monitoring of the temperature of the line and the fluid conditions within the cryogenic fluid housing be maintained during the time that the plug is being formed and maintained. Premature thawing of a temporary plug of the type described above could produce catastrophic results if, for example, the plug was formed to retain a volatile or environmentally damaging fluid within the line during the repair or modification process. Accordingly, it has become desirable and necessary to provide means for remote control and monitoring of the temperature of the line exterior surface, which temperature is indicative of the condition of the frozen plug within the line. It is also important to be able to monitor the flow of cryogenic fluid to the aforementioned housing during the formation and maintenance of the temporary plug to efficiently utilize the cryogenic fluid and prevent excessive consumption of fluid in the formation and maintenance of the plug.